As discovered many decades ago, L4 engines begin to experience severe vibration when the displacement passes about 2.2 liters (134").
Above that, light reciprocating bits and a large rod ratio help. Balance shafts frequently used in modern engines.
The L4 crank is much stiffer than the L6, and doesn't have harmonic problems. The L6 is not quite in perfect balance, since rocking couple will always exist between cylinders, there's no way to place the counterweight exactly 180° from (directly opposite) the crank throw.
